1. Field of the Invention
The present invention relates in general to in-line type electron guns for color picture tubes and, more particularly, to an improved structure in such in-line type electron guns for prevention of electron beam spot distortion at screen comers due to influence of deflection magnetic fields of a deflection yoke placed around the neck of the color picture tube.
2. Description of the Prior Art
As well known to those skilled in the art, the focus characteristics of the typical color picture tubes are potently influenced by apertures of electron lenses or main lenses of the electron guns. Here, the electron lenses or the main focusing lenses of electron guns of the color picture tube comprise a plurality of anodes. It is preferred to enlarge the apertures of the main lenses of the electron guns for achieving excellent focus characteristics of the color picture tube. In the color picture tube, three electron guns may be arranged in the delta type or in the in-line type. In the typical in-line type electron guns, three electron guns corresponding to three colors, that is, red (R), green (G) and blue (B) are arranged in a horizontal line, thus to be integrated into the in-line type electron guns. With the above structure comprising the three electron guns, the in-line type electron guns which are to be arranged in a limited space inside the neck of the color picture tube should be undesirably limited in both the apertures of main focusing lenses of the three electron guns and the intervals between the main focusing lenses. In this regard, the typical in-line type electron guns of the color picture tube should have a serious problem in enlarging of the main lens apertures of its three electron guns for achieving the excellent focus characteristics of the color picture tube.
The above problem will be described in detail in conjunction with the accompanying drawings.
With reference to FIG. 1, there is shown in a plan section view a color picture tube having a typical in-line type electron guns. The glass envelope of the color picture tube is designated by the numeral 1. A fluorescent screen 3 for developing a color image is mounted on the inner surface of a face plate 2 of the glass envelope 1. The screen 3 is applied with three color phosphors in the form of vertical stripes on its surface. Here, the three color vertical phosphor stripes are alternately applied on the screen 3. The center axes 15, 16 and 17 of three cathodes 6, 7 and 8 of the in-line type electron guns are placed in a horizontal line such that they are parallel with each other. The three center axes 15, 16 and 17 are aligned with centers of their respective openings of the first control grid or G.sub.1 electrode 9, the second control grid or G.sub.2 electrode 10, the second anode or G.sub.3 electrode 11 of the electron lens. The in-line type guns also includes a shielding cup 13 in from of the first anode or of a G.sub.4 electrode 12. A contact spring (not shown) is mounted on a side of the shielding cup 13. The G.sub.3 electrode 11 cooperates with the first anode or the G.sub.4 electrode 12 so as to form the electron lenses or the main lenses of the electron guns. This G.sub.4 electrode 12 has three openings in which the center opening is concentric with the center axis 16 of the cathode 7. However, the centers 18 and 19 of opposed openings of the G.sub.4 electrode 12 are eccentric from the center axes 15 and 17 of the cathodes 6 and 8 respectively. The electron beams produced by the three cathodes 6, 7 and 8 travel along their center axes 15, 16 and 17 so as to be received by the main lenses.
As best seen in FIG. 2, the main lenses of the electron guns comprises the two anodes, that is, the G.sub.3 electrode 11 used as a focusing electrode and the G.sub.4 electrode 12 used as an accelerating electrode. The two electrodes 11 and 12 comprise electrode envelops respectively. The electrode envelops of the electrodes 11 and 12 have their burring sections 116 and 126 each of which defines a common opening for the three electron beams and which extend to a predetermined length in opposed directions from inside edges of elliptical rims 115 and 125. The extending portion of the burring sections 116 and 126 are parallel with the outer surfaces of envelops of the electrodes 11 and 12 respectively. The elliptical tract type rims 115 and 125 of the two electrodes 11 and 12 face each other and have a predetermined width.
In FIG. 2, please let the X-X' direction, which is perpendicular to the axial direction or the beam travelling direction, be the "horizontal direction" and let the Y-Y' direction, which is perpendicular to the horizontal direction X-X', be the "vertical direction". The electrodes 11 and 12 include their control electrode plates or planes 113 and 123 that are placed in the envelops of the electrodes 11 and 12. "Plates" or "planes are used interchangeably in the present specification. The control electrode plates 113 and 123 are recessed from the rims 115 and 125 by a predetermined distance in the axial direction. The electrode plates 113 and 123 are adapted to control the electron beams. Such an electrode plate 113 or 123 has an elliptical center opening 119 or 129 whose vertical (or Y-Y' directional) size is larger than the horizontal (or X-X' directional) size. The opposed sides of each electrode plate 113 or 123 are defined by concave edges such as formed by vertically cutting the centers of elliptical openings. The opposed sides of the plate 113 or 123 thus form opposed side openings in cooperation with the envelope of electrode.
The electrical potential of G.sub.3 electrode 11 is lower than that of the G.sub.4 electrode 12. The higher potential of the G.sub.4 electrode 14 is equal to those of the shielding cup 13 and of a conductive layer 5 applied on the inner surface of the glass envelope 1. Conventionally, the voltage applied to the G.sub.3 electrode 11 is about 20%-30% of that applied to the G.sub.4 electrode 12. Since the center openings 119 and 129 of the electrode plates 113 and 123 are coaxial with each other, the main lens formed in the center axis of the electrodes 11 and 12 is axially symmetrical. In this regard, the center electron beam focused by the main lens travels along the center beam path coinciding with the center axis 16.
Meanwhile, the opposed side openings of the G.sub.3 electrode 11 are eccentric from those of the G.sub.4 electrode 12, so that each of the side lenses formed at the opposed sides of the electrode 11 and 12 is axially asymmetrical. At the divergence sections of the side lenses formed at the G.sub.4 electrode 12, the side beams thus pass through the side beam paths deflecting from the center axes of the side lenses toward the center beam path. At this time, the side beams are influenced by focusing of the side lenses and converged to the center beam path. The three electron beams are thus converged to a shadow mask 4 for color selection and image-produced thereon. The shadow mask 4 is disposed in the envelop 1 of the color picture tube such that it is spaced apart from the fluorescent screen 3. The above convergence of the three electron beams is so-called static convergence (hereinbelow, referred to simply as "STC"). At the shadow mask 4, only the components, which excite the color phosphor stripes corresponding to the respective electron beams subjected to the color selection of the shadow mask 4, are transmitted through the shadow mask 4 so as to reach the fluorescent screen 3. In the color picture tube, the electron beams 23 are scanned on the fluorescent screen 3, so that the electron beams 23 should be deflected to the screen comers using outside magnetic fields. The above object is achieved by a deflection yoke 14 which is placed on the glass envelope 1 about the neck and forms the outside magnetic fields, that is, the horizontal magnetic field and the vertical magnetic field, in the color picture tube.
In the above in-line type electron guns, the main lenses common to the three electron beams of the three cathodes are more influenced by the vertical focusing/accelerating electric field than the horizontal focusing/accelerating electric field. Each of the electron beams out of the main lenses thus shows an elliptical section whose horizontal diameter is longer than its vertical diameter. In order to compensate such elliptical shapes of the electron beams out of the main lenses, the control electrode plates 113 and 123 having their elliptical openings are placed in the envelops of the G.sub.3 and G.sub.4 electrodes 11 and 12 such that they are recessed from the rims 115 and 125 by the predetermined distance in the axial direction. In the elliptical openings of the control electrode plates 113 and 123, the vertical (or Y-Y' directional) diameter is longer than the horizontal (or X-X' directional) diameter and this specified structure of the elliptical openings compensates the elliptical shape of the electron beams. With the above structure of the main lenses of the in-line type electron guns, the STC which is an important characteristic of each side beam is determined by the recessed distances of the control electrode plates 113 and 123 from the rims 115 and 125. In addition, the main lenses have a difference between the horizontal convergence and the vertical convergence of the electron beams. Such a difference is so-called astigmatism.
Such an astigmatism is produced at the center of the typical in-line type electron guns on purpose. That is, the electron beams 23 scanned on the fluorescent screen 3 are influenced by the deflection magnetic fields of the deflection yoke 14, thus to be deflected to the screen comers. In this regard, the vertical convergence of the electron beams is strengthened but the horizontal convergence of the beams is weakened and this causes distortion of the electron beams. In order to prevent such a distortion of the electron beams, the typical in-line type electron guns produce the astigmatism at the center, thus to compensate the distortion of the electron beams about the screen comers. However, the generation of the astigmatism for compensation of the distortion does not completely removes the distortion but still remains the distortion of the electron beams at both the screen center and the screen comers. This causes distortion of the screen and deformation of the control electrode plates of the electron lens of the electron guns.